The requirement for developing high power density, high efficiency and low profile DC/DC converters and DC/AC inverters has exposed a number of limitations in the use of conventional wound-wire magnetic structures. A number of high frequency (HF) power transformers have been developed, such as conventional E core or pot core HF power transformers (first generation), planar core power transformers (second generation) and coaxial core power transformers (third generation).
The planar core and coaxial magnetic core structures exhibit many advantages such as their suitability for high frequency operation, high power density and small physical size. A smaller physical size is achievable with coaxial magnetic core structures because no heat sink is required by the coaxial magnetic core, which makes the actual converter size much smaller than the planar core.
The planar core and coaxial magnetic core structures also exhibit high efficiency, lower losses due to eddy currents and improved thermal control, the latter because the cooling surfaces on both the inner coil surface and the outer core surface are larger. The planar core and coaxial magnetic core structures further exhibit a low electromagnetic interference (EMI) problem, low leakage inductance and low coupling capacitance between the windings. Thus, planar core and coaxial magnetic core structures are chosen for HF power transformers in energy conversion systems.
However, in high frequency (HF) applications up to 1 MHz, the inter-winding capacitance couples HF noise from the primary winding to the secondary winding and causes serious common mode HF noise problems, as described by L. Tihanyi, Electromagnetic Compatibility in Power Electronics, Piscataway, N.Y., IEEE, 1995, pp. 143-146. The effect of such parasitic capacitances can not be neglected if the operating frequencies are above 100 kHz.
One attempted solution to this problem is the insertion of Faraday shields between the primary and secondary windings to suppress the HF noise coupled to the secondary winding. However, eddy currents are generated in the Faraday shields, which produce a heating effect, lead to demagnetization and reduce performance. Reference may be had to U.S. Pat. No. 6,420,952 B1 assigned to Core Technology Inc. and entitled Faraday Shield and Method as an example of a planar transformer including a Faraday shield between the primary and secondary windings. The Faraday shields in this patent comprise a plurality of low conductivity areas in the form of holes to restrict or inhibit the flow of eddy currents in the Faraday shields. However, it has been found that these Faraday shields still produce a heating effect leading to demagnetization, which prevents optimum performance being achieved.
Another problem with many prior art planar transformers is that they comprise many external connections between the multiple layers, which can be prone to damage.
Hence, there is a need to further reduce as much as possible the electromagnetic compatibility (EMC) and EMI problems of the prior art without increasing the eddy currents and/or address or at least ameliorate one or more of the other problems of the prior art.
In this specification, the terms “comprises”, “comprising”, “includes”, “including” or similar terms are intended to mean a non-exclusive inclusion, such that a method, system or apparatus that comprises a list of elements does not include those elements solely, but may well include other elements not listed.